Before Zenda (Spoiler Warning)
It's been mentioned how similar to the Prisoner of Zenda this film is, but there is an earlier Colman film that is even nearer the story.
In The Masquerader (1933) Colman plays a British politician who is unable to continue functioning because of drug taking (!) His doppleganger cousin (played also of course by RC) takes his place in parliament and of course is a resounding success.
Like in Dave, the original dies and the stand-in gets the wife.